1. Field of the Invention
The invention is generally related to the control of subsea equipment and more particularly to a power module for control of hydrocarbon production equipment in deep water.
2. General Background
In the drilling and production operations for hydrocarbons offshore, it is necessary to position equipment such as a blowout preventer or subsea tree at or near the sea floor. Three different types of control systems have been accepted and used in offshore drilling and completion operations.
The direct hydraulic control system was the first system ever used offshore. All power for the controls is located above the water surface, with hydraulic lines that lead down to the equipment at the sea floor. Its advantage is high reliability, independent control of selected functions, and relatively low installation cost. The main disadvantage is slow response time, which makes it unsuitable for deep-water applications that require fast response times.
The piloted hydraulic control system uses pilot line pressure to open and close small volume valves that control flow from high-pressure accumulators. The flow from accumulators operates blowout preventers or other valves on the ocean floor. This system uses a smaller control bundle and operates much faster than a direct hydraulic control system. The system operates faster because it uses smaller volumes and it dumps the excess fluid at the ocean floor after each function is performed. The main advantages are speed and reliability. The umbilical line is smaller, takes up less room on the rig, and costs less than a direct hydraulic umbilical line. The piloted system performs well up to about three thousand feet of water depth. The disadvantages are that the system requires accumulators to function and the number of accumulators needed increases as the water depth increases. The system requires a hydraulic supply line to recharge the accumulators after operating the system because it dumps the fluid at the ocean floor.
The electro-hydraulic control system operates solenoid valves to direct high pressure or high volume from the supply accumulators. The supply accumulators will operate blowout preventers or valves on the ocean floor. The advantages are fast operation and a small umbilical with one supply line. The disadvantages are that the system requires accumulators to function and the number of accumulators increases as the water depth increases. The electro-hydraulic control system works in deep water but requires a very large volume of accumulators. This system requires a hydraulic supply line to recharge the accumulators after operating the system. It also dumps its hydraulic fluid at the ocean floor after functioning.
The present state of the art requires huge numbers of accumulators to provide hydraulic power controls for the deeper water depths that have become more common place in drilling/producing hydrocarbons. The present state of the art also presents a potential pollution problem when non-biodegradable hydraulic fluids are used. Thus, it can be seen that the present state of the art leaves a need for a means of supplying hydraulic power to sub sea controls at deeper water depths that does not require an accumulator volume that increases with water depth and that does not present pollution concerns.